Disc drive systems have been used in computers and other electronic devices for many years for storage of digital information. Information is recorded on concentric memory tracks of a magnetic disc medium, the actual information being stored in the form of magnetic transitions within the medium. The discs themselves are rotatably mounted on a spindle, the information being accessed by means of transducers located on a pivoting arm which moves radially over the surface of the disc. The read/write heads or transducers must be accurately aligned with the storage tracks on the disc to ensure proper reading and writing of information; thus the discs must be rotationally stable.
Electric spindle motors of the type used in disk drives conventionally rely on ball bearings to support a rotary member, such as a rotating hub, on a stationary member, such as a shaft. Ball bearings are known to wear parts, and in time increased friction will cause failure of the motor. In addition, ball bearings create debris in the form of dust or fine particles that can find their way into “clean” chambers housing the rotary magnetic disks which are driven by the motor. The mechanical friction inherent in ball bearings also generates heat, noise and vibration, all of which are undesirable in a disk drive motor.
Fluid bearings represent a considerable improvement over conventional ball bearings in spindle drive motors. In these types of systems, lubricating fluid, either gas or liquid, functions as the actual bearing surface between a stationary base or housing in the rotating spindle or rotating hub of the motor. Liquid lubricants, for example, oil, complex ferro-magnetic fluids or even air, have been utilized in hydrodynamic bearing systems. As compared with ball bearings, fluid dynamic bearings have improved running accuracy, greater impact strength and lower noise generation.
One example of a spindle motor utilizing a prior art fluid dynamic bearing is disclosed in U.S. Pat. No. 5,658,080. The '080 patent shows a spindle motor including a shaft retained in a shaft retainer and a thrust plate provided to the shaft. A thrust dynamic pressure fluid bearing is provided between the thrust plate and the shaft retainer. A radial dynamic pressure fluid bearing is provided between the shaft retainer and the shaft. A ring-shaped space formed between the outer circumferential surface of the thrust plate and the inner circumferential surface of the shaft retainer is partially filled with oil. Ring-shaped projections are provided on the outer circumferential surface of the thrust plate. When the motor is running at a high-speed, oil in the ring-shaped space is retained on the shaft retainer and through this oil, oil in the thrust dynamic pressure fluid bearings at the upper and lower surface sides are communicated with each other.